[An(acac)4] - complexes revisited


[An(acac)4] - complexes revisited

Gericke, R.; Kaden, P.

Actinides (An) play an important role in chemical research and environmental science related to the nuclear industry or nuclear waste repositories.1 Investigating their coordination chemistry can function as a tool to obtain fundamental understanding of actinide binding. Due to the radiotoxicity of actinide complexes, special care in handling those material need to be employed in form of working in a controlled area lab. Therefore, the understanding of complexation properties of the actinides, in particular the transuranium (TRU) elements, is lacking behind those of the d- or 4f-elements, which can be handeled in ordinary laboratories.
For the early actinides possible oxidation states are typically ranging from +II to +VII. A suitable approach to explore fundamental physico-chemical properties of the actinides is to study series of isostructural An compounds in which the An is in the same oxidation state.2 Therefore our investigations are directed towards the synthesis of actinide complexes (An = Th, U, Np and Pu) with the f-element in the oxidation state IV, the dominant oxidation state particularly under anoxic environmental conditions. Observed changes in e.g., the binding situation or magnetic effects along such a series deliver insight into the elements’ unique electronic properties mainly originating from the f-electrons. One important question in the field of An chemistry is the degree of “covalency” in compounds across the An series,3 which may be addressed by systematic studies on series of An compounds, including transuranium (TRU) elements.
An-complexes using pure O-donor ligand systems can act as molecular mimic for related An-O-systems, e.g. UO2 used as fuel in nuclear reactors. In these studies, we investigate the coordination chemistry of tetravalent actinides (An(IV)), using an organic monoanionic ligand with O-donor atoms of the acetylacetonate (acac) type. Since 1958, actinide complexes of the type [An(acac)4] have structurally been caracterized at ambient temperature.4-8 However, spectroscopic data is limited to vibrational spectroscopy especially for the transuranium complexes, leaving open questions of the actinide bonding. The [An(acac)4] complexes are typically synthesized via salt metathesis reactions under strict exclusion of moisture and air. Single crystal X-ray diffraction analysis at 100 K provides insight into isostructural complex series, which were achieved in each case. In order to obtain further insight into the electronic structure of these complexes, the compounds were further analysed by NMR, IR, UV-vis-NIR, and EPR spectroscopy. The redox chemistry of the series of [An(acac)4] complexes in NCMe was further investigated with cyclic voltammetry. These results are used as a basis to further analyse bonding trends along the actinide series by means of quantum chemical calculations.
From the results, trendlines along the actinides An = Th, U, Np and Pu in this complex series were obtained, which shed some light in the ongoing debate of covalency in actinide bonding.

References
1. L. S. Natrajan, A. N. Swineburn, M. B. Andrews, S. Randall, S. L. Heath, Coord. Chem. Rev. 2014, 266-267, 171-193.
2. M. B. Jones, A. J. Gaunt, J. C. Gordon, N. Kaltsoyannis, Chem. Sci. 2013, 4, 1189-1203.
3. M. P. Kelley, J. Su, M. Urban, M. Luckey, E. R. Batista, P. Yang, J. C. Shafer, J. Am. Chem. Soc. 2017, 139, 9901-9908.
4. D. Grdenić, B. Matković, Nature 1958, 182, 465-466.
5. D. Grdenić, B. Matković, Acta Cryst. 1959, 12, 817-817.
6. B. Allard, Acta Chem. Scand. 1972, 26, 3492-3504.
7. D. Brown, B. Whittaker, J. Tacon, J. Chem. Soc., Dalton Trans. 1975, 1, 34-39.
8. B. Allard, J. Inorg. Nucl. Chem. 1976, 38, 2109-2115.

Keywords: actinides; NMR; EPR; single crystal X-ray diffraction; quantum chemical calculations

  • Lecture (Conference)
    Actinides revisited 2022, 21.-23.09.2022, Dresden, Germany

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Publ.-Id: 35385